Rhodococcus equi is a Gram-positive, facultative intracellular coccobacillus classified in the order of Acitnomycetales. It is primarily a soil organism. It has been recognised as a positive agent of a debilitating and potentially fatal bronchopneumonia affecting foals worldwide. R. equi is considered to be one of the most significant pathogens in the equine breeding industry.
The successful early diagnosis and treatment of Rhodococcus equi in foals and management of the foals environment to reduce the risk of contracting the disease are, arguably, among the most challenging experiences currently facing equine stud farms. Presently the treatment of R. equi disease is by the prolonged administration of a combination of antimicrobials, macrolides, i.e. erythromycin, azithromycin or clarithromycin, and rifampicin. However, as this therapy risks antibiotic resistance and adverse drug reactions in the foal and the dam, improved means of therapy and prophylactic treatment are required.
R. equi can also affect non-equine species. In pigs R. equi is associated with granulomatous lymphadenitis of cervical lymphatic tissue and in man R. equi can cause cavitary pneumonia, predominantly in immunocompromised individuals especially those with acquired immune deficiency syndrome (AIDS). As a consequence of the AIDS pandemic, R. equi pneumonia has become a disease of increasing significance in human medicine. R. equi infections have also been described in cattle, sheep, goats, lama, cats and dogs, but disease in these species is rare with lesions confined to lymph node abscessation or wound infection.
Infection by R. equi relies on the ability of R. equi to colonise the airways and replicate inside macrophages which is dependent on its capacity to interfere with endosomal maturation following phagocytosis and to prevent acidification of the vacuole in which it resides. Eventually, intracellular proliferation of the pathogen leads to the necrotic death of the marcophages accompanied by massive damage to lung tissue characterised by cavitation and granuloma formation.
Studies of the virulent strains of R. equi have determined that such strains possess an extra chromosomal DNA element known as a plasmid, which is associated with virulence. Plasmids isolated from regular strains infecting foals have been proposed to include a region that represents a pathogenicity island, which is a DNA fragment containing genes required for virulence. The pathogenicity island identified contains a family of nine virulence associated protein (Vap) chains (VapA-VapC-Vap-I, pseudo-VapE). Killed/inactivated R. equi organisms do not illicit protective immunity, and there is no consistent evidence that protein or DNA vaccines, based on the highly immunogenic VapA surface antigen, are efficacious in producing protection against a Rhodococcal pneumonia in foals. In view of the lack of an efficacious vaccine, R. equi infection is a major cause of mortality in young foals and the heavy economic losses incurred due to R. equi has a major economic impact in countries where thoroughbred racing and breeding is important (USA, Australia, Ireland, Argentina, UK, France, Spain, Germany, Austria, Japan etc.). There is a need for treatment regimes and a vaccine to be developed which can be used to control R. equi on farms, in particular stud farms.